The present invention relates to gas flow arc interruption devices, and, particularly, to nozzle configurations for such devices.
During the interruption of alternating current electrical circuits carrying high currents, the formation of an arc is generally used to provide a medium which can carry the high values of current near the peak of the sinusoid but convert rapidly to an effective insulating medium at times of current zero. Accordingly, a large effort has been expended over the years to design arc interruption devices which extinguish these arcs at current zero as quickly as possible. In particular, efforts have been expended in the development of gas-blast interrupters in which a pressurized blast of air, SF.sub.6, or other cooling gas is blown either through or around the arc in attempts to cool the arc or to change its position or shape. Cooling of the arc is necessary to reduced the kinetic motion of the ionized arc constituents so as to promote recombination and elimination of the hot plasma gases which sustain the arc.
In a large class of arc interruption devices, electrodes are disposed along the axis of a gas-blast nozzle. In one such design, generally referred to as a single-flow design, the gas blast flows in a single direction from a region near the upstream electrode toward a downstream electrode. In another related design, generally referred to as a dual-flow design, the gas blast is introduced through ports in the side of the nozzle into a central nozzle region between two axially-disposed electrodes. In this design, the flow is outward from this central portion toward each of the electrodes. In this design, each electrode can be said to be a downstream electrode.
For both the single-flow and dual-flow designs just described, several attempts have been made to determine which axial region of a convergent-divergent gas-blast interrupter nozzle contributes most strongly to the thermal arc recovery process. In a nozzle with a high pressure ratio and small divergent angle between the downstream nozzle walls, some researchers have concluded that turbulent cooling of the arc well downstream of the nozzle throat leads to rapid thermal recovery in that region. See "Investigation on the Physical Phenomena Around Current Zero in HV Gas Blast Breaker", IEEE Transactions on Power Applications and Systems, Volume 95, pages 1165-1176 by D. Hermann et al., (1976).
Other design efforts in this field, participated in by the instant inventor, have indicated that, for an orifice nozzle, or a nozzle with a high divergence angle operated at a moderate pressure ratio between inlet and outlet pressures, the flow pattern includes a strong shock wave on the nozzle axis downstream of the throat. Such a shock wave produces rapid deceleration of the gas flow from supersonic velocity to subsonic velocity. When an arc is present along the nozzle axis, the shock wave interacts with the arc to broaden it. Furthermore, efforts have shown that the broadened arc downstream of the shock wave contributes very little, if anything, to the thermal arc recovery process. This conclusion is in agreement with an earlier analysis which indicated that the time constant for thermal cooling of an arc near current zero scales as arc diameter squared. See "Uber das AbKlingen von Lichtbogen. I", Zeitschrift fur angewandte Physik, Volume 12, pages 231-237 by G. Frind (1960).
Nozzles which are designed to operate at high upstream-to-downstream pressure ratios with a small divergence angle in the downstream section can maintain shock-free supersonic flow for relatively long distances downstream of the throat. Such nozzles may be able to derive a significant recovery speed contribution from this region. However, such nozzle designs are difficult to use in practice because the flow is easily constricted, or blocked, by either the downstream electrode (in the case of a downstream contact withdrawn through the nozzle throat during interruption) or the arc itself at high currents.
Several interrupter designs known in the art have employed two convergent-divergent nozzles in series, with the gas-blast flow directed radially inward to an upstream stagnation point on the axis between the two opposed nozzles, then in opposite axial directions through the nozzles. Gas-blast circuit breakers of this kind are described, for example, in Richter et al. U.S. Pat. No. 3,739,124, issued Jun. 12, 1973, in H. O. Noeske U.S. Pat. No. 3,739,125, issued Jun. 12, 1973, and in Tokuyama et al. U.S. Pat. No. 3,996,439, issued Dec. 7, 1976. Although such designs, commonly referred to as dual-flow interrupters, achieve two subsonic-to-supersonic flow transition points on a single arc, they still develop shock waves in their downstream nozzle sections and loose potential recovery speed from the resulting broadened arc.